Mike,

There is a philosophy conversation behind your question having to do with risk acceptance.  Some people feel having a separate start bank is essential for redundancy.  Some (like myself) feel a combined bank is fine.  Factors include the kind of cruising you do (coastal/coastwise/blue water), equipment aboard (genset), and availability of towing services (BoatUS/SeaTow).  I'm sure you'll find both points-of-view here on T&T.  That's ultimately a choice based on your personal preferences.

In terms of the DC electrics on your boat, it is certainly viable to combine your banks into one, single bank.  If you observe the 50% state of charge maximum rule, you will still have plenty of power to start the engine.  You have two 645 aHr banks, so 645 aHr of usable capacity battery.  At 50% discharge, you would still have 300 aHr or reserve.  If you need around 500 amps to run your starter motor, and you crank for 5 seconds (a very long time), you use 500 * (5/3600) = 0.7 amp hours of energy.  If you need 1000 amps of that same 5 seconds, then you'd use 1.4 aHr to start that engine.  Should be PLENTY of power.

If you do have a separate start bank, you are generally better off to use batteries designed for start service.  But there are tradeoffs.  Start service batteries would be smaller, but they do have different charging needs than deep cycle house banks.  Wiring is more complex and switching is subject to memory failures in the human computer located between the owner's ears.  Lots of pros and cons.  I consolidated my banks, and am very happy with that decision.

I posted an article on this subject on my personal website, here: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/battery-topics/battery-bank-separate-vs-combined/ https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/battery-topics/battery-bank-separate-vs-combined/.

Hope this is useful.

Jim

Peg and Jim Healy aboard Sanctuary, currently at Charlotte Harbor, Punta Gorda, FL
http://gilwellbear.wordpress.com http://gilwellbear.wordpress.com/
Monk 36 Hull #132
MMSI #367042570
AGLCA #3767
MTOA #3436

Jim,

Your discussion on minimizing gassing to reduce water use in valid,
however, it overlooks one important factor, stratification.

Trojan states on their Q & A website:
(http://www.trojanbattery.com/tech-support/faq/)

"Can I reduce my maintenance by not gassing my flooded batteries?
You will reduce the frequency of watering, but will cause a condition
known as stratification where the specific gravity of the electrolyte is
light at the top of the battery and heavy at the bottom. This condition
results in poor performance and reduced battery life."

There are many write-ups on stratification on the web so I will not go
into detail on the subject here.  Basically, stratification is where the
acid separates out so that the solution at the bottom of the battery is
very strong and that at the top is nearly water.  The strong acid
corrodes the plates and shortens the battery life.

Agitating the electrolyte will prevent or remove stratification. This
can be done by physically agitating the battery, or more effectively by
establishing some gassing so that the rising bubbles tend to mix the
electrolyte.  Interestingly, in certain applications where large banks
of batteries are in standby service such as in a central telephone
office, they often have special batteries with tubing built into the
bottoms of the cells that permit them to bubble an external gas through
the cells to prevent stratification.

It should be noted that most modern batteries have a special type of
cell cap that is designed to recover much of the water normally lost in
the out flowing gas.

It might also be noted that Trojan specifies a somewhat higher
adsorption voltage that typical, I suspect this is again an effort to
insure gassing and the subsequent de-stratification. Another place on
their Q & A website they say: "It is completely normal for flooded
batteries to lose water. They lose water because of the gassing that
they go through with every charge. Gassing is an important part of the
recharge process. The gassing allows the electrolyte to mix well and
helps the battery get back to a fully charged state at the end of the
charge process. Unfortunately, the gassing also causes water to be lost."

So bottom line, in the end, efforts to minimize gassing to prevent water
loss may be self defeating.

Tom Collins - Misty Sea